Field of the Invention
present invention relates to two-dimensional infrared solid-state imaging elements employing a thermal type infrared detector for detecting and absorbing incident infrared radiation and converting the same into heat.
A thermal type infrared detector is a device which temperature is raised upon irradiation of infrared radiation by absorbing the irradiated infrared radiation that further performs detection of temperature changes.
FIG. 11 is a bird""s-eye view showing an example of an arrangement of a single pixel of two-dimensional infrared solid-state imaging elements employing a conventional thermal type infrared detector utilizing a thermal type thin-film which resistance value is changed depending on temperature.
In the drawing, 1 denotes a semiconductor substrate comprised of semiconductors of e.g. silicon, 10 an infrared detector section being disposed in a spaced relationship with respect to the semiconductor substrate 1, 11 a thermal type thin-film, 21, 22 supporting legs for lifting and holding the infrared detector section 10 above the silicon semiconductor substrate, 31, 32 metallic wirings for supplying current to the thermal type thin-film, 40 a switching transistor for switching between ON and OFF of current running through the thermal type thin-film 11 and the metallic wirings 30, 31, 60 a control clock wire for controlling ON and OFF conditions of the switching transistor, and 70 a metallic reflecting film for forming an optical resonance structure with the detector section in order to increase the absorption of infrared radiation at the infrared detector section 10.
FIG. 12 is a view showing a sectional arrangement along current paths of the structure of a pixel of the two-dimensional solid-state imaging elements employing a conventional thermal type infrared detector as shown in FIG. 11 wherein the switch-transistor 40, signal wire 50 and control clock wire 60 are omitted since these are not directly concerned in the present invention.
As already mentioned, the thermal type thin-film 11 is formed above the infrared detector section 10 wherein the metallic wiring 31, 32 are connected to the thermal type thin-film 11 and further connected via contact portions 122, 122 to a signal read out circuit (not shown) formed on the silicon semiconductor substrate.
The thermal type thin-film 11 and metallic wiring 31, 32 are covered by insulating films 100, 110 of silicon dioxide film or silicon nitride film wherein these insulating films 100, 110 constitute the mechanical structure of the infrared detector section 10 and supporting legs 21, 22. 80 denotes an insulating film for insulating the signal read out circuit and the wiring 31, 32 that are formed on the semiconductor substrate 1, and the light detector section 10 is disposed above the metallic reflecting film 70 above the insulating film 80 with a hollow section 90 being interposed therebetween. Another insulating film may be formed on the surface of the metallic reflecting film 70.
Next, operations of conventional two-dimensional solid-state imaging elements employing such a thermal type infrared detector will be explained.
Infrared radiation is made incident from a side at which the light detector section 10 is disposed and is absorbed by the light detector section 10.
Owing to the presence of the metallic reflecting film 70, stationary waves of incident infrared radiation wherein the position of the metallic reflecting film 70 forms a node are formed, and by suitably setting the distance between the infrared detector section 10 and the metallic reflecting film 70, absorption of infrared energy can be increased in the infrared detector section 10.
Infrared energy that has been absorbed at the infrared detector section 10 is converted into heat and increases the temperature of the infrared detector section 10. The degree of temperature rise is dependent on the amount of incident infrared radiation (while the amount of incident infrared radiation is dependent on the temperature and thermal emissivity of an object to be picked up).
Since the degree of temperature rise can be known by measuring a change in resistance values of the thermal type thin-film 11, the amount of infrared radiation that is emitted by the object to be picked up can be known from changes in resistance values of the thermal type thin-film 11.
As a material for the bolometer that exhibits large changes in resistance owing to changes in temperature, semiconductors of vanadium oxide (VOx) or the like may be employed as known from reference P. W. Krise, xe2x80x9cUncooled IR Focal Plane Arraysxe2x80x9d, Proceedings of SPIE, vol. 2552, pp. 556-563.
In case resistance temperature coefficients of thermal type thin-films 11 are identical, the larger the temperature rise of the infrared detector section 10 is, the larger the change in resistance that is obtained by an identical amount of incident infrared radiation becomes, and the higher the sensibility becomes. In order to increase the degree of temperature rise, it is effective to reduce the amount of heat escaping from the infrared detector section 10 to the silicon semiconductor substrate 1 as little as possible, and due to this fact the supporting legs 21, 22 are designed as to limit thermal resistance as much as possible.
It is also important to set a thermal capacity of the infrared detector section 10 small such that a temperature time constant of the infrared detector section 10 becomes smaller than a frame time of the imaging elements.
While infrared radiation is made incident into entire pixels, only those that are made incident into a portion of the infrared detector section 10 contribute to the temperature rise of the infrared detector portion 10 (although some amount of infrared radiation that is made incident into the supporting legs which are close to the infrared detector section 10 are also effective), and infrared radiation that is made incident into remaining regions become ineffective.
Due to this fact, it can be easily understood that it is also effective to increase an aperture ratio (a ratio of an area of the infrared detector section 10 with respect to an area of the pixel) for increasing the sensitivity.
In a method for detecting changes in temperature by using a borometer as explained above based on a conventional example, it is necessary to employ a material of large change in resistance caused by temperature and low noise such as vanadium oxide (VOx) that is usually not used in a silicon process.
While such a material can be treated in film-forming, photolithograpy or etching processes using similar manufacturing techniques as known for silicon processes, it has been difficult to perform processes in manufacturing lines that are used for silicon VLSI in view of contamination of silicon processes.
Further, in the arrangement of the conventionally known infrared solid-state imaging device as shown in FIG. 11 and FIG. 12, the infrared detector section 10 needs to be formed at most on a region other than the supporting legs 21, 22 and contact portions for connecting these supporting legs and the read out circuit that is formed on the silicon semiconductor substrate 1, whereby the aperture ratio was restricted by the design of the supporting legs, contact portions and interval clearance between these portions and the infrared detector section 10 such that high sensitivity could not be obtained.
Such problems became more remarkable the smaller the pixels were so that it was difficult to obtain high resolution using small pixels while maintaining proper sensitivity.
The present invention has been made in view of the above problems, and it is a purpose of the present invention to provide infrared solid-state imaging elements which are two-dimensional infrared solid-state imaging elements that form a thermal type infrared detector on a same semiconductor substrate as a signal read out circuit is formed, wherein all processes except for a final process of eliminating a sacrificial layer (in case an underlying layer is etched and an overlying is maintained, the underlying layer that is removed is generally called a sacrificial layer) can be performed in a conventional silicon VLSI manufacturing line, and wherein it can be realized for a thermal type infrared detector which is capable of achieving a high aperture ratio without being dependent on the design of supporting legs, metallic wiring or contacts which constitute a heat insulating structure, whereby it is enabled to provide infrared solid-state imaging elements which can be obtained through simple manufacturing processes and which exhibit high sensitivity.
Infrared solid-state imaging elements of the present invention are comprised with an infrared absorbing section that is formed as to correspond to each pixel aligned in a two-dimensional pattern for absorbing incident infrared radiation and converting the same into heat, a temperature detector section that is formed as to correspond to each pixel on a semiconductor substrate and are arranged of a plurality of serially connected silicon pn junction diodes that are biased in a forward direction, a hollow section formed on each region on which the temperature detector sections is formed on the semiconductor substrate, supporting mechanisms that are arranged of materials exhibiting large thermal resistance and which support the temperature detector portion above the hollow section on the semiconductor substrate, and a joint column for thermally coupling the infrared absorbing section and the temperature detector section. With this arrangement, all of manufacturing processes except for eliminating sacrificial layers can be performed in a silicon VLSI process line, and due to the fact that active elements other than silicon pn junction diodes used in the temperature detectors can be eliminated out of pixel portions, infrared solid-state imaging elements can be manufactured in a stable manner which exhibit improved productivity, which are of low cost, and which are highly uniform.
Further, the arrangement of the infrared absorbing section and temperature detector section as separated layers and the provision of the joint column which is a means for mechanically and thermally coupling the infrared absorbing section and temperature detector section, the area of the infrared absorbing section which practically determines the aperture ratio can be increased to thereby obtain high aperture ratio and high sensitivity.
Also, according to the infrared solid-state imaging elements of the present invention, by employing a SOI substrate as the semiconductor substrate, the silicon pn junction diodes for detecting the temperature can easily be formed by using crystal Si as a constituent member.
The silicon pn junction diodes of the temperature detector section for the infrared solid-state imaging elements of the present invention are formed in that a plurality of silicon pn junction diodes are arranged by alternately forming a p-layer and n-layer on a single crystal silicon layer, and in that the diodes are connected through metallic wiring between connections in a reverse direction at the time of applying voltage. With this arrangement, silicon pn junction diodes can be disposed at high density within a restricted region of an area for the pixels, and the number of silicon pn junction diodes can be increased to thereby achieve high sensitivity.
According to the infrared solid-state imaging elements of the present invention, platinum silicide that is formed in a self-aligned manner at an aperture portion is used as metallic wiring for short-circuiting of the wiring whereby simplification of processes can be achieved.
According to the infrared solid-state imaging elements of the present invention, a p-type semiconductor substrate is employed as the semiconductor substrate and the plurality of serially connected silicon pn junction diodes of the temperature detector section-that are biased in a forward direction are formed within n-type impurity region layers that are formed on the p-type semiconductor, whereby no insulating film is required below the temperature detector section but it can be coped with electrolytic etching so that conventional substrates that are cheaper than SOI substrates can be employed as semiconductor substrates.
According to the infrared solid-state imaging elements of the present invention, the infrared absorbing section is formed of an infrared absorbing metallic thin-film, insulating layer and a metallic reflecting film whereby it can be achieved for improving absorption of infrared radiation by arranging the infrared absorbing section to be thin and to be of an interference absorbing structure, and thus for achieving high sensitivity.
According to the infrared solid-state imaging elements of the present invention, the infrared absorbing section is formed of an insulating layer and a metallic reflecting film, a process of forming an infrared absorbing metallic thin-film can be eliminated such that simplification of manufacturing processes can be achieved.
According to the infrared solid-state imaging elements of the present invention, since the joint column is formed of a part of composition members of the infrared absorbing section, the joint column can be simultaneously formed with the infrared absorbing section such that simplification of manufacturing processes can be achieved.
Further, according to the infrared solid-state imaging elements of the present invention, it has been provided for an etching stop layer exhibiting etching resistivity against an etchant for etching the hollow section at peripheral portions of a region-for forming the hollow section within the semiconductor substrate. With this arrangement, there is no fear that etching is unnecessarily spread, margins between structures that are to be formed on regions to be etched and regions not to be etched can be made small, and high densification of silicon pn junction diodes for temperature detection is enabled owing to the gained regions for the temperature detector section.
Further, since the distance between each of the pixels can be made small, the pixels can be consequently made smaller such that the small sized pixels can be disposed at high density.
Also, according to the infrared solid-state imaging elements of the present invention, constant-current sources one end of which is connected to a fixed potential are provided for each vertical line. With this arrangement, the constant-current sources provide load for detecting output signals for each of the vertical lines, and time for electric conduction can be set to be longer for each single pixel even though the number of pixels is increased whereby reading out of signals can be satisfactorily performed and noise of output signals can be decreased by providing narrow bandwidths.
According to the infrared solid-state imaging elements of the present invention, resistances one end of which is connected to a fixed potential are provided for each vertical line. With this arrangement, the resistances provide load for detecting output signals for each of the vertical lines, and time for electric conduction can be set to be longer for each single pixel even though the number of pixels is increased whereby reading out of signals can be satisfactorily performed and noise of output signals can be decreased by providing narrow bandwidths.
According to the infrared solid-state imaging elements of the present invention, diodes one end of which is connected to a fixed potential are provided for each vertical line. With this arrangement, the diodes provide load for detecting output signals for each of the vertical lines, and time for electric conduction can be set to be longer for each single pixel even though the number of pixels is increased whereby reading out of signals can be satisfactorily performed and noise of output signals can be decreased by providing narrow bandwidths.
According to the infrared solid-state imaging elements of the present invention, the diodes which one end is connected to a fixed potential for each vertical line are arranged in that the same number of diodes of identical shape are serially connected as the silicon pn junction diodes of the temperature detector section for the pixels. With this arrangement, characteristics are varied similarly to silicon pn junction diodes of the temperature detector section in accordance to changes in temperature of the pixels such that the compensation of changes in output owing to changes in pixel temperature is enabled.
According to the infrared solid-state imaging elements of the present invention, each of the vertical lines are provided with common load one end of which is connected to a fixed potential via a horizontal selective transistor, whereby nonuniformity in output signals owing to nonuniformity in load for each vertical line can be eliminated.
According to the infrared solid-state imaging elements of the present invention, the common load with respect to each of the vertical lines are diodes arranged in that the same number of diodes of identical shape are serially connected as the silicon pn junction diodes of the temperature detector section. With this arrangement, nonuniformity in output signals owing to nonuniformity in load for each vertical line can be eliminated and characteristics are varied similarly to silicon pn junction diodes of the temperature detector section in accordance to changes in temperature of the pixels such that the compensation of changes in output owing to changes in pixel temperature is enabled.